A new technique has been developed to include horizontal and deviated well depths in geostatistical velocity simulations for layer-cake depth conversion. This method uses a multi-pass approach to generating the realizations. First, the velocities for all of the layers are simultaneously simulated at the horizontal and deviated well locations. The velocities honor the "hard" velocity information from the vertical wellbores, the top depths from the deviated wells, and the "less than" depths imposed by the horizontal well data. It can also honor secondary information, such as isochrons or horizon times which are correlated with velocity. In the second pass, the simulated velocities at the deviated and" horizontal well locations are treated as "hard" data points and are incorporated to create multiple simulation maps of the velocity field throughout the area.
The impact of this method is that the horizontal and deviated well information will constrain the structural uncertainties of the top reservoir surface. This information impacts reserves above a gas/water contact and constrains the top reservoir surface to reduce uncertainty at nearby drilling locations. In addition, this technique constrains the spatial variability of the velocity field in the layers above the horizontal and deviated control points.
Application of this method of depth conversion to a field in the Southern Gas Basin of the North Sea will be shown and pros and cons illustrated.
The Excalibur field is part of MNSL's Greater Lancelot Area located in Quad 48 Blocks 17 and 18 in the UK Southern North Sea (Fig. 1). The Lancelot 3D survey covers the Excalibur field, three producing gas fields, and four additional gas accumulations. Fourteen vertical appraisal wells and 12 horizontal development wells have been drilled within the survey area (Fig. 2).
The reservoir is in the Lower Permian Rotliegendes sandstone. It lies unconformably upon the Carboniferous and consists of an eolian and fluvial-dominated sand sequence. The reservoir thickness varies between 200-600 feet, increasing to the Northwest. In Excalibur field, the thickness averages 275 feet. The Rotliegendes is overlain by the Upper Permian Zechstein evaporite sequence of alternating high velocity (approximately 20,000 feet/sec) anhydrites and dolomites and slower velocity (approximately 14,500 feet/sec) halites.
The Southern North Sea is an area well known for lateral stratigraphic changes causing lateral velocity variations which require a detailed depth conversion process to produce accurate depth maps. A series of Jurassic-Triassic grabens have been filled with large amounts of slower velocity Jurassic sediments. These grabens cut across the survey area creating lateral velocity variations (Fig. 3). In addition, changes in the thickness of the Zechstein sequence also cause variations that need correction.
The depth conversion process utilized is vertical and has four layers of a "layer-cake". The interval time for each layer is converted to thickness using interval velocity maps. The vertical wells provide "hard" control points for all layers whereby, assuming well times match seismic times, the depth maps will tie the well tops. A deviated well will encounter each layer at a different XY location; therefore, the depth points in the layers above are estimates with uncertainty depending on the angle of deviation and proximity to vertical well control (Fig. 4). The challenge is accurately predicting velocity and depth away from the well control in the shallower layers so that the deviated wells tops tie the depth map at the reservoir layer without having to introduce anomalous apparent velocities in the last layer or a flexing filter to force the surface to tie the wells as a final step.
To meet this challenge. we were looking for a way to utilize the horizontal well information to constrain the velocity modelling in the shallow layers.